The present invention relates to a process and apparatus for the conversion of solid, hydrocarbonaceous materials such as coal to a more valuable gaseous product. In particular, the present invention relates to a fluidized bed coal gasification reaction wherein coal is gasified and by-product ash is efficiently withdrawn.
As natural gas and crude oil supplies become uncertain, it has become necessary to search for alternative energy sources. Because of its ready availability in the United States, coal has increasingly been looked at as an alternate energy source for natural gas and crude oil. Unfortunately, however, much of the coal in the United States has a high sulfur content which, when burned directly, can lead to substantial atmospheric pollution and acid rain. By way of example, it has been estimated that the combustion products of coal contribute one-eighth of the total atmospheric pollutants emitted in the United States including one-half of the sulfur oxides and one-fourth of both the nitrogen oxides and particulate matter.
Sulfur emissions from coal combustion may be reduced by several methods. These methods include using low sulfur coal; cleaning high sulfur coal by physical methods to remove the sulfur from the coal; removing sulfur from the coal during the combustion thereof; producing a de-ashed low sulfur solid fuel by the solvent processing of coal; and, lastly, gasifying coal and removing the sulfur from the resultant gas prior to combustion of the gasified coal products.
The last method, coal gasification with cleaning of the resultant gas products prior to combustion, appears to offer the greatest reduction in sulfur emissions since most of the sulfur present in the gasified coal appears as hydrogen sulfide. The removal of this hydrogen sulfide, however, from the gasified coal, presents no great problem since several different commercial gas cleaning processes are available today which can reduce the hydrogen sulfide content of a gaseous stream, such as produced in a coal gasification reaction, to less than 10 ppm. In fact, some processes can produce gaseous streams containing hydrogen sulfide of 1 ppm or less.
A preferred method for the gasification of coal is the U-GAS Process developed by the Institute of Gas Technology in Chicago, Ill. (See the Oil and Gas Journal--Aug. 1, 1977, p. 51 et seq., the teachings of which are incorporated herein by reference). The U-GAS Process is capable of producing a clean, environmentally acceptable low BTU (about 150-300 BTU/SCF) fuel gas from coal. This gas can be used directly by industrial and commercial users or as a substitute for natural gas or fuel oil. In the form of synthesis gas, the products from the U-GAS Process can be used as a chemical feedstock or as a source of hot reducing gas for reducing metallic ores such as iron ore to the base metal. In this latter application, it is desirable to have a high ratio of carbon monoxide and hydrogen to steam and water in the hot product gases because of the high reducing properties of carbon monoxide and hydrogen.
In the U-GAS Process, the gasification reaction is performed at high temperatures since this maximizes the production of carbon monoxide and hydrogen. Preferred gasification temperatures for the U-GAS Process are in the range of 1500.degree. to 2000.degree. F. and preferably 1600.degree. to 1900.degree. F. Lower temperatures are not desirable since this leads to the production of high amounts of carbon dioxide and water. However, one of the potential problems encountered in the high temperature gasification of coal in any gasification process including the U-GAS Process is the fusion of ash particles at the high temperatures encountered in the gasification reaction. These high temperatures cause the ash particles to become sticky and agglomerate within the reaction zone. Accordingly, although temperatures in excess of 1700.degree. F. are desirable for coal gasification, it is difficult to substantially exceed 1950.degree. F. since temperatures substantially in excess of 2000.degree. F. lead to the formation of sticky ash particles that can agglomerate to form large ash particles that are difficult to remove from the fluid bed.
One method of removing agglomerated ash particles from a fluid bed reactor, the basic principles of which are used in the U-GAS Process, is illustrated in Jequier et al, U.S. Pat. No. 2,906,608, the teachings of which are incorporated by reference herein. In this apparatus, an inverted conical withdrawal section is positioned in the bottom of the fluid bed reactor to provide a venturi-type nozzle having a constricted center section. A high velocity air-steam stream is passed up through this inverted conical section and reacts with coal therein to create locally higher temperatures within the confined cone positioned at the bottom of the reactor. Within this inverted cone the ash particles are heated to temperatures sufficient to render them sticky whereby they gradually agglomerate and become larger in mass and size. When they reach a predetermined value, size and/or weight, the velocity of the gas stream rising up through the cone becomes insufficient to keep these agglomerated particles in the fluid bed and the particles descend down through the narrow bottom portion of the inverted cone and are withdrawn from the fluid bed reaction zone in a relatively efficient manner. Because the velocity of the gaseous material passing up through the cone always exceeds the settling velocity of the finely divided coal particles in the fluid bed per se, the agglomerated ash particles can be selectively removed without removal of the coal particles from the fluidized bed proper.
A problem associated with a venturi-type apparatus, as illustrated in Jequier et al, is that extremely high temperatures are present in the conical withdrawal section. For example, the temperatures within the conical withdrawal zone are at least 100.degree. and often 200.degree. higher than the temperatures encountered in the fluid bed proper. Since the abrasive agglomerated ash particles are in constant physical contact with the walls of the cone and because of the high temperatures present therein, exotic expensive alloys are required to manufacture a long lasting withdrawal cone. Most importantly, since the gas stream that forms the ash agglomerates is the same as the stream separating or classifying the agglomerates from the fluidized bed, unusual restrictions are imposed on the rate and composition of gas flow. In addition, sintering can take place in the venturi and plugging of the nozzle can occur particularly when fine coal material recovered from the product gases are recycled back to the fluidized bed through the venturi nozzle. Because the plugging occurs in a zone of high temperature, a fused adherent mass can form and lead to an undesired premature reactor shutdown.
Chen et al, U.S. Pat. No. 3,981,690 teaches the undesirability of utilizing a venturi nozzle such as Jequier et al in a coal gasification process and, instead, suggests a process for gasifying coal in a narrow, spout fluidized bed wherein air entering a central tube is contacted with feed coal in an annular section at the bottom portion of a relatively small diameter reactor. Ash is formed in the bottom of the reactor and removed downward through the annulus. This method of simultaneous coal addition and ash withdrawal does not recognize the necessity of providing an introduction point separate from the fresh coal feed point, the importance of the location of the central tube relative to the fluid bed and the ash withdrawal annulus, and the importance of controlled, oxygen concentration at the bottom of the fluidized bed including high oxygen concentrations near the central tube to provide efficient ash agglomeration and withdrawal.